The goals of the present proposal are designed to develop an understanding of the mechanisms and cytology of early female mammalian meiosis. It is planned to investigate (1) the number and location of chiasmata in diplotene oocytes from fetal material of 5 species to answer fundamental questions about sexual dimorphism, inter- and intrachromosomal interference and terminalization and aging effects of disjunction, (2) to develop a technique for banding of early female meiotic stages suitable for specific idenfication of each bivalent, (3) to develop pachytene maps and analyses of the major structural rearrangements reported thus far in mammals and (4) to study the mechanisms of the switch from mitosis to meiosis in the female fetal gonad and its subsequent inhibition using an in vitro system.